This invention relates generally to gas turbine engines, and more specifically to methods and systems for operating gas turbine engines at significantly lower load conditions while having stable combustion and lower emissions.
Gas turbine engines typically include a compressor, one or more combustors each having a fuel injection system, and a turbine section. In an engine having a plurality of combustors, they are typically arranged in an annular array about the engine and most typically interconnected for the purposes of ignition. The compressor raises the pressure of inlet air, and then directs it to the combustors, where it is used to cool the combustion chamber walls as well to provide air for the combustion process. In the combustion chamber, compressed air is mixed with a fuel and the mixture is ignited by an ignition source to produce hot combustion gases.
While a full load condition is the most common operating point for land-based gas turbines used for generating electricity, often times electricity demands do not require the full load of the generator, and the operator desires to operate the engine at a lower load setting, such that only the load demanded is produced, thereby saving fuel costs. Combustion systems of the prior art have been known to become unstable at lower load settings while also producing unacceptable levels of carbon monoxide (CO) and oxides of nitrogen (NOX) at these lower load settings, especially below 50% load. This is primarily due to the fact that most combustion systems are staged for most efficient operation at high load settings and therefore operate less efficiently at lower load settings. Furthermore, lower emissions can be achieved through premixing air and fuel together prior to combustion, instead of through diffusion, and therefore premixing generally facilitates combustion for lowest NOX emissions. However, even with various methods of fuel staging, known gas turbine engines are still limited in turndown capability.
The combination of potentially unstable combustion and higher emissions often times prevents engine operators from running engines at lower load settings, forcing the engines to either run at higher settings, thereby burning additional fuel, or shutting down, and thereby losing valuable revenue that could be generated from the part-load demand. A further problem with shutting down the engine is the additional cycles that are incurred by the engine hardware. A cycle is commonly defined as the engine passing through the normal operating envelope and thereby exposing the engine hardware to a complete cycle of pressures and temperatures that over time cause wear to the engine hardware. Engine manufacturers typically rate hardware life in terms of operating hours or equivalent operating cycles. Therefore, incurring additional cycles can reduce hardware life requiring premature repair or replacement at the expense of the engine operator.